Nanogap Arrays And Their Plasmonic Resonance Properties

From over half a century ago till now,people's attention on the nanoscale materials keeps increasing,due to their change of surface/internal atom arrangement and the ultrahigh specific surface area,thus exhibiting widely different properties compared with the corresponding bulk materials.Among the various nanomaterials,noble metal nanostructures could efficiently interact with light to generate unique surface plasmon resonance(SPR),exciting giant enhancement of local electric field,readily enabling sensing,trace detection,miniaturization of optical devices and biotherapy.Nevertheless,most of the researches in this field remain in the experimental stage.Promoting nanomaterials from laboratory to practice requires not only the tailored nanostructures with strong and controllable SPR to provide as efficient and stable sensing platform but also the low cost,high throughput and high integration nanofabricating method to meet the industrial production level.Recently,the development of nanogaps makes one see the dawn to simultaneously satisfy the property and fabrication requirements.Introducing nanoscale gaps to the noble metal nanostructures can induce strong plasmonic coupling of the bilateral metals to obtain much enhanced electric field intensity inside the nanogap.When the gap size gradually approaches the limit of conventional fabrication methods of sub-10 nm,ultra-strong local electric field could be excited to realize single molecule detection.So nanogap structures with strong plasmonic coupling can readily satisfy the property requirement to promote nanomaterials from lab to practice;while for the fabrication requirement,the nanometric gaps have already touch the resolution limit of conventional nanofabrication method.To develop appropriate fabrication methods for larger-area,higher integration and stronger coupling nanogaps to meet industrial production is still challenging.Aiming at addressing these challenges,we work on developing emerging nanofabrication methods to build novel nanogap arrays.By actively combining simulations and experiments,the mechanisms and origins of the strong electric-magnetic coupling induced by nanogaps were systematically investigated.Moreover,we innovatively propose the concept of chiral nanogap for the first time,and further build a bridge from chiral plasmonic nanostructures to complex chiral sensing applications?In Chapter 2,simple and versatile colloidal lithography is invoked to fabricate large-area-ordered metal-insulator-metal(MIM)multilayer nanohole arrays.The thickness of spacer film SiO2 could be accurately tuned in nanometric resolution.This kind of planar nanogap layer can induce strong electric-magnetic coupling between upper and lower nanohole arrays.Thus the MIM nanohole arrays with optimized structural parameters could excite multiple strong Fano resonances based magnetic modes,to obtain sharp SPR peaks,which is also confirmed through FDTD simulations.Due to the sharply reduced Fano-featured SPR peak widths,they always possess ultrahigh sensing sensitivity even to the smallest change of the surroundings.Based on this,electrochromic polymer film is spin-coated onto the MIM nanohole array.Real-time and reversible tuning of the Fano-featured SPR peak over a large range was realized by controlling the refractive indices of the wrapping polymer film with applied voltage.The pure electric-induced SPR shift(decoupling from the polymer absorption)reaches up to 72 nm,which is the highest value ever reported,thanks to the ultrahigh sensitivity of the narrow Fano-featured SPR peak.In clear contrast to other electro-tuned SPR devices,ours is ITO-free due to the excellent inherent conductivity of the continuous MIM nanohole film.The saving of the ITO mediate for electron transfer facilitates the direct communication between electro-active polymer and the MIM nanohole array,hence accelerates the switching process,promoting the realization of real-time sensing and even remote sensing.We believe the proposed fast-responsive MIM nanohole/electro-active polymer nanocomposite will play an active role in tunable lasers,optical modulators and photovoltaic device.In Chapter 3,our attention is moved from the planar nanogaps to the vertically aligned nanogaps.For most of the optically detecting instruments,the incident light is normal to the substrates.Thus,comparing with the planar nanogaps,vertical ones could more efficiently interact with incident light,to maximize the electric field intensity inside the gaps and increase the sensing ability.However,building sub-10 nm vertical nanogaps has always been a great challenge in nanofabrication field.To overcome this barrier,we jump out of the conventional fabrication methods,and develop a young nanofabrication method,nanoskiving,to generate millimeter-long one dimensional(1D)linear nanogaps with low cost,high throughput,superb repeatability but no expensive instruments.The gap-width could be preciously tuned in sub-10 nm range with nanometric resolution.Hence the electric field intensity is optimized in both simulation and experiment.When the gap-width is set at 5 nm,the strongest SPR is excited owing to the formation of plasmonic standing waves.Further and importantly,to keep up with the rapid development of modern integrated optical/electrical technology,we organically combine nanoskiving with surface patterning techniques to fabricate larger-area,highly-integrated two-dimensional(2D)nanogap arrays.Not contented with this,we explore further and realize the breakthrough from 2D planar integration to 3D steric integration,to successively construct “multistorey building-like” 3D nanogap arrays by taking advantages of the unique transferability of nanoskiving.For the first time,we realize the integration improvement of nanogaps by taking adequate use of vertical space.Moreover,to our surprise,the “multistorey building-like” 3D nanogap arrays not only enable higher integration,the electric field intensity at the crossing points is also significantly enhanced due to the adiabatic plasmonic resonance.And the smaller stacking angle contributes to even drastically larger electric field enhancement,which could support ultrasensitive surface enhanced Raman scattering(SERS)sensing.The highest electric field intensity reaches up to 20000,which is rarely seen in relevant reports.The novel 3D nanogap array will certainly broaden the view of fabricating 3D complex nanostructures,and provide theoretical and experimental foundation for understanding the enormous electric field intensity.In Chapter 4,based on the strongly coupled 3D nanogap array,the model is specialized to investigate chiral optical response of the skew-stacked 3D chiral nanogaps,and then promote its practice application in chiral sensing field.Due to the unique transferability and precious controllability of nanoskiving,we could perform beyond conventional top-down and bottom-up methods to pioneer an edge lithography method via nanoskiving to fabricate chiral nanostructures with low cost,high throughput,superb resolution together with outstanding design freedom,providing a brand new avenue to complex chiral nanostructures.Notably,the chiral nanostructures fabricated through nanoskiving are equipped with nanoscale gaps,enabling strong electric-magnetic coupling and rapidly enhanced local electric field intensity,which is in clear contrast with the ones from conventional fabrications.Benefiting from the bi-functions of prominent chiral response and strong plasmonic coupling in the novel 3D nanogap structure,we just use Raman spectrometer to directly realize enantiodiscrimination in absence of any chiral illuminant and chiral label molecules for the first time.The non-polarized light from the general Raman spectrometer interacts with the specific chiral nanogaps,exciting local chiral electric field with specific chirality,to either favor or hinder the Raman scattering of the specific chiral molecules.The Raman intensity of different-handed chiral molecules differ a lot,enabling fast and reliable enantiodiscrimination.